1. Field of the Invention
The present invention relates to an electrode material, a method of manufacturing the electrode material, an electrode, and a nonaqueous electrolyte battery.
2. Description of the Related Art
A nonaqueous electrolyte battery using lithium as a negative electrode active material is recently noticed as a high energy density battery. A primary battery using manganese dioxide (MnO2), carbon fluoride [(CF2)n], thionyl chloride (SOCl2) or the like as a positive electrode active material has been already utilized widely as a power source for a pocket calculator or a watch, and a backup battery for a memory. Recently, along with the trend in reduction of size and weight of electronic appliances such as VTR and communication equipment, a secondary battery of high energy density is demanded as a power source, and a lithium secondary battery using lithium as a negative electrode active material has been intensively researched.
A lithium secondary battery having the configuration explained below is being studied. That is, a negative electrode contains lithium. A nonaqueous electrolysis solution is prepared by dissolving a lithium salt such as LiClO4, LiBF4, or LiAsF6 in a nonaqueous solvent such as propylene carbonate (PC), 1,2-dimethoxy ethane (DME), γ-butyrolactone (γ-BL), or tetrahydrofuran (THF). Instead of the nonaqueous electrolysis solution, a lithium conductive solid electrolyte may be also used. A positive electrode contains a compound which performs topochemical reaction with lithium as a positive electrode active material. Examples of the compound include TiS2, MOS2, V2O5, V6O13, and MnO2.
However, the above-described lithium secondary battery has not been realized yet. The main reason is that the lithium for use in the negative electrode is pulverized in the repeated process of charge and discharge to become reactive lithium dendrite, thereby lowering the safety of the battery or even leading to breakage, short-circuiting or thermal runaway of the battery. In addition, due to deterioration of the lithium, the efficiency of charge and discharge is lowered, and the cycle life becomes shorter.
Accordingly, instead of the lithium, it is proposed to use a carbonaceous material which intercalates and deintercalates lithium, such as a coke, a baked resin, carbon fibers, and pyrolytic vapor phase carbon. A recently commercialized lithium ion secondary battery comprises a negative electrode containing a carbonaceous material, a positive electrode containing LiCoO2, and a nonaqueous electrolyte. In such a lithium ion secondary battery, owing to the recent demand for further reduction in size of electronic appliances or extended use time, it is required to further enhance the charge and discharge capacity per unit volume, and the development is in progress, but it is insufficient. It is hence needed to develop a new negative electrode material for realizing a battery of high capacity.
As a negative electrode material capable of obtaining higher capacity than a carbonaceous material, it is proposed to use single metals such as aluminum (Al), silicon (Si), germanium (Ge), tin (Sn) and antimony (Sb). In particular, when Si is used as a negative electrode material, a high capacity of 4200 mAh per unit weight (1 g) is obtained. In the negative electrode using such a single metal, however, the metal is microscopically pulverized by repetition of lithium insertion and deinsertion, so that a high charge and discharge cycle characteristic cannot be obtained.
To solve these problems, it has been attempted to enhance the charge and discharge cycle life by using an alloy containing element T1 which does not form an alloy with lithium and element T2 which forms an alloy with lithium as negative electrode materials. Examples of the element T1 which does not form an alloy with lithium include Ni, V, Ti and Cr. To suppress pulverization which causes to deteriorate cycle characteristic, for example, volume expansion is suppressed by dispersing a phase that is active with lithium (for example, element T2 phase) and a phase that is inactive with lithium (for example, element T1 phase) in nano scale. Or, to suppress the pulverization, an amorphous alloy is used as the negative electrode material.
In any negative electrode material mentioned above, lithium is absorbed in the negative electrode material by causing an alloying reaction between the negative electrode material and lithium. An example of a first charge reaction is shown in the following formula (A).T1xT2y+Li→xT1+LiT2y  (A)
After the first charge and discharge reaction, second and subsequent charge and discharge reactions will proceed in the reaction shown in the following formula (B).xT1+LiT2yLi+yT2  (B)
Since the reaction does not progress completely reversible in the second and subsequent reaction process (B), Li is collected in the alloy, and as the cycles are repeated, lithium supply source is lost, so that the cycle cannot be repeated. In an amorphous alloy, the reaction progress is very smooth at an initial stage. However, as cycles are repeated, crystallization is progressed, and cycle deterioration occurs.
In addition, the negative electrode material causing an alloying reaction with lithium at the time of charge is high in reactivity with a nonaqueous electrolyte containing a nonaqueous solvent such as ethylene carbonate, and a film of Li2CO3 or the like is formed on the surface of the negative electrode by a reaction between lithium in the negative electrode material and the nonaqueous electrolyte. Therefore, coulomb efficiency of the negative electrode during a charge and discharge cycle is lowered. Further, in the case of using as a positive electrode active material a Li-containing composite oxide such as LiCoO2, Li in the positive electrode active material comes into shortage as the charge and discharge cycle progresses, and thus, obvious capacity deterioration is observed.
An example of the negative electrode material causing an alloying reaction with lithium at the time of charge is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-311681. This publication discloses a negative electrode material for a lithium secondary battery containing particles mainly composed of an amorphous Sn.A.X alloy of a non-stoichiometric composition (A denotes at least one transition metal; X denotes at least one selected from the group consisting of O, F, N, Mg, Ba, Sr, Ca, La, Ce, Si, Ge, C, P, B, Bi, Sb, Al, In, S, Se, Te and Zn; and the ratio of atomic number is Sn/(Sn+A+X)=20 to 80 at. %). In the amorphous alloy disclosed in the publication, absorption and release of lithium occurs due to the alloying reaction with lithium. Therefore, the irreversible capacity is likely to occur by repetition of charge and discharge cycles, and the charge and discharge cycle life becomes short.
Another example of the negative electrode material is mentioned in pages 430 and 431 in Collected Papers of 44th Battery Symposium in Japan (Nov. 4, 2003). This document describes that Yuichi AKASAKA of Tottori University and others synthesized a LixCeSn3 compound by mechanical alloying, and attempted to use the compound in a negative electrode of a lithium secondary battery. The Collected Papers also report that, when lithium is added to a base alloy (CeSn3) by mechanical alloying, the volume change due to lithium absorption and release in the obtained alloy is lessened.
However, in the LixCeSn3 alloy mentioned in the paper, the charge and discharge cycle life is only about 10 cycles at most, as shown in FIG. 3.
Jpn. Pat. Appln. KOKAI Publication No. 2003-346793 discloses realization of a lithium secondary battery in which high discharge capacity and excellent cycle characteristic have been achieved by paying attention to an RSn3 phase (R=a rare earth metal element) having a strong polarity and resistance to pulverization. The negative electrode of the lithium secondary battery uses an alloy having a basic skeleton of RSn3—Lix(0≦x≦13) in which Li amount x is correspond to a remaining amount of Li in the alloy caused by first charge and discharge cycle. The alloy is prepared by high frequency induction heating. An embodiment of Jpn. Pat. Appln. KOKAI Publication No. 2003-346793 describes that, in order to obtain the RSn3 phase, a transition metal element such as Co, Ni, Fe, Cu, V and Cr is not added at all, or added only very slightly. Jpn. Pat. Appln. KOKAI Publication No. 2003-346793 also mentions that the initial discharge capacity is extremely lowered when the contents of such transition metal elements are increased.
However, the alloy containing the RSn3 phase disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-346793 is very large in volume change due to lithium absorption and release, as with the alloy mentioned in the above collected paper, and therefore, a sufficient charge and discharge cycle life is not obtained.